This invention relates to a printing plate for all-over printing of large areas by the intaglio printing process, to a method for producing the printing plate, and to a data carrier with a large-area printed image produced by the intaglio printing process.
In line intaglio, flat representations are known to be produced by closely adjacent engraved lines, the individual engraved lines normally being fractions of a millimeter wide and separated from each other by unengraved lands.
For the printing operation the engraved lines of the printing plate are filled with ink. Surplus ink is removed from the printing plate with the aid of a wiping cylinder or doctor blade such that the engraved lines are filled with ink up to the edge. The lands separating the engraved lines are cleaned in this operation at the same time.
During printing, finally, the data carrier to be printed, generally paper, is pressed onto the printing plate under high pressure by means of a pressure cylinder having an elastic surface. The data carrier is thereby pressed into the ink-filled engraved lines of the printing plate, thereby coming in contact with the ink. When the data carrier is detached it draws the ink out of the depressions of the engraved lines. The resulting printed image has printed lines which vary in ink layer thickness depending on the depth of the engraving.
If one uses translucent inks in line intaglio one obtains light tones when printing a white data carrier with low ink layer thicknesses, and darker tones when printing with thick ink layers.
In comparison with other common printing processes, the intaglio printing process can produce printed images with very great ink layer thicknesses. The resulting printed images are even perceptible manually if the engravings are deep enough. By using accordingly fine engravings, however, one can also obtain extremely fine and sharp printed lines.
Although the intaglio printing process can produce very high-quality printed images resolved into line structures, it has the disadvantage of not being able to produce large continuous printed areas, i.e. lines with a width of about one millimeter and more. This is because when the inked printing plate is wiped, not only the surplus ink is removed in the area of large-area engravings but also ink from the engraving. This lowers the ink surface below the surface level of the printing plate in said engraved areas. Since the paper pressed into the engraved areas of the printing plate does not reach the ink surface in all places, gaps arise in the printed image which render the print useless.
The problem of the present invention is therefore to provide measures which permit large-area printed image areas to be printed all over by the intaglio printing process so as to produce a uniform color effect for the viewer.
This problem is solved according to the invention by the features of the independent claims. Developments are to be found in the subclaims.
The invention starts out from the finding that one can prevent ink from being wiped out of the area of the engraving when the printing cylinder or plate is wiped by providing so-called separating lands or partitions in the engraving which prevent: or minimize the action of the wiping cylinder on the ink incorporated in the printing plate engraving. It is suspected that the wave of surplus ink pushed over the printing plate surface by the wiping cylinder during wiping draws parts of the ink out of the engraving as well by reason of hydrodynamic effects. The partitions apparently prevent ink in the engraving from being moved within the total volume and entrained with the wave of ink of the wiping cylinder. The partitions thus divide a large-area engraving into contiguous xe2x80x9cchambersxe2x80x9d or channels which permit ink to be taken out perpendicular to the printing plate surface during printing but not during wiping parallel to the printing plate surface.
The partitions are preferably disposed transversely to the direction of rotation of the printing cylinder. In this arrangement they apparently cause a shearing of the wave of ink during the wiping process and thus a hydrodynamic decoupling of ink in the engraving from the wiping process taking place on the printing plate surface.
In cases where it is not possible to arrange the partitions transversely to the wiping direction, the partitions at least effect a division of large-area engravings, giving them a similar function with respect to wiping out of ink as exists with fine-structured engravings.
Taking the basic inventive idea into account in optimized form, the engraved areas are preferably to be equipped with partitions transversely to the wiping direction. For engraved lines extending along the wiping direction this yields a division of the engraved lines into adjacent partial portions. The engravings extending transversely or diagonally to the wiping direction are divided at least in the longitudinal direction of the engraved line, the partitions preferably extending parallel to the engraving edges.
In cases where the engraving not only consists of very wide engraved lines but also contains large-area engraved elements having similar extensions in the x and y directions, it is also possible to execute the partitions as a screen, i.e. to provide intersecting partitions extending e.g. lengthwise and crosswise with respect to the wiping direction. It is also possible to provide partitions in the form of concentric circles in a honeycomb shape or the like. Such a formation of the partitions not only has the advantage of in any case guaranteeing the function of the partitions independently of the wiping direction, but also ensures that the partitions have increased mechanical stability.
Inventively providing partitions in the engraving of the intaglio printing plate already proves especially advantageous as of an engraved line width greater than 0.5 millimeters. For engraved lines with a width of one millimeter and more they prove to be almost imperative.
The height of the partitions can be varied within a relatively great span, as tests have shown. If the partitions end at the level of the printing plate surface one should make sure that the partition form, viewed in cross section, tapers in a wedge shape. This ensures that the engraving is divided into separate channels or chambers in the optimum form, on the one hand, while the sharp-edged partitions cause no interruption of the printing area on the other hand.
If one lowers the upper partition edges below the level of the printing plate surface, the cross-sectional form of the partitions can deviate from the wedge form almost at will, i.e. be trapezoid, rounded or a different shape. Since the upper partition edges are always disposed below the level of the printing plate surface in this case and thus always covered with ink, the production of a continuous printing area is ensured in any case.
It has turned out that when one uses partitions whose upper edges end exactly on the level of the printing plate surface the surface of the wiping cylinder wears out relatively quickly. Lowering the upper partition edge by at least 2 microns to 5 microns eliminates this problem. Such a minimum lowering is in any case recommendable for this reason.
Tests have furthermore shown that much greater lowering of the upper partition edges is also possible. A lowering of up to about 50% below the level of the printing plate surface, based on the engraving depth, is accordingly possible.
It has also turned out that if the partitions have a height (also referred to in the following as amplitude) based on the engraving depth of more than 50%, they cause xe2x80x9cnotchesxe2x80x9d in the ink layer surface on the thus produced printing area. Although the printing area produced with such a large-area engraving is printed continuously with ink, it thus has a surface relief caused by the partitions. The surface relief is especially pronounced if the partition amplitude is selected in the range of 75% to 100% of the engraving depth. At lower amplitudes, e.g. in the range of about 60% this surface relief becomes ever weaker, finally disappearing completely at an amplitude of about 50%. Below the value of 50% one must increasingly expect printing errors in the form of gaps or skips rendering the print useless, particularly with deeper engravings.
Tests have finally shown that engraving depths of 5 microns to about 150 microns are excellent to use according to the invention. The preferred engraving depth found for the production of common printed images was the range of 10 microns to 60 microns. Using customary intaglio printing inks, one thus obtains ink layers with a rather translucent color effect, and even slight changes in engraving depth lead to readily visible changes in tone. Engravings with a depth in the range of about 60 microns to 100 microns are particularly suitable for printing ink layers with a saturated, opaque color effect. The exact values of course vary depending on whether a light or dark color is involved.
Engravings with a depth of 100 microns and more are particularly suitable for producing ink layer structures with a relief readily perceptible to the touch.
The finer the fine structure of the printed area represented by the surface relief is, the less it appears when viewed without aids (magnifying glass). This applies at least to fine structures resulting from partitions with-a distance of about 20 microns to 150 microns and a wedge shape. Partitions with a distance of 150 microns to about 400 microns are already recognizable with the naked eye, but in no way disturb the flat general impression of the printed color area. If one uses a trapezoid cross-sectional profile instead of wedge-shaped partitions, the notches in the surface relief become wider, i.e. more real. Such structures permit a creative influence on the area to be printed since e.g. the screen formed by the partitions also appears as a layout element. If the partitions are not worked into the engraving like a screen but in the form of characters, graphic symbols or the like, these characters or graphic symbols are also recognizable in the printed area.
If one enlarges the partition distance clearly above 500 microns, the above-mentioned printing errors in the form of ink gaps, skips, spots or the like increasingly occur.
Considering that the production of intaglio printing plates is already one of the most elaborate methods for producing printing plates, it is easy to see that additionally providing partitions in the engraving raises considerable additional problems. This holds all the more since not only the form, amplitude and arrangement of the partitions ate necessary for the inventive function, but also a precision in the micron range. Such printing plates are not producible manually or by means of etching. The inventive prints and printing plates therefore ensure a high measure of protection against forgery and imitation.
However, such printing plates can be produced by an engraving apparatus from the applicant, as described in WO 97/48555. This apparatus makes it possible to mill intaglio printing plates by computer control. The lines of a two-dimensional linework are detected by a computer and the area of each individual line exactly defined. Using an engraving tool, e.g. a rotating chisel or laser beam, the outside contour of these areas is first engraved to cleanly border the area. Subsequently the bordered region of the area is cleared out using the same or another engraving tool so that the total line is exactly engraved according to the line original. Depending on the nature and control of the engraving tool one can produce both a certain roughness (instead of a smooth surface) on the base of the engraving, and the inventive partitions with any desired amplitude, different flank angles or precisely given cross-sectional forms. The important thing, as mentioned above, is that the partitions have a minimum amplitude of about 50% of the engraving depth for the inventive function. If this value is fallen clearly short of, ink adheres to the base of the engraving better than with a smooth engraving base, but the abovementioned printing errors are inevitable with large-area engraved elements.
The invention offers completely new possibilities of design for intaglio printing plates. By using engravings printing over large areas it is now possible to produce engraved lines with a width of 1 millimeter to 10 millimeters and more, with ink layer thicknesses of 40 microns and more. One can also realize continuous geometric areas with a size of a few square centimeters by intaglio printing without problem The fine structure of the printing area can be present both in the form of a screen and in the form of characters or graphic symbols. Even if the coarsest fine structure is selected (with a partition distance in the order of magnitude of 500 microns), it cannot be imitated with any known printing process, which considerably increases the forgery-proofness of accordingly printed data carriers. The fine structure thus proves not only the use of the intaglio printing process, which is already high-quality itself, but also the use of the engraving apparatus described in WO 97/48555, which is not available to any forger because of the high costs.